MySQL split up multiple columns into multiple rows

Question

I have a customer supplied dataset which looks like this: (simplified)

|ForiegnKey|Code1|Code2|Code3|  
|==========|=====|=====|=====|  
|A         |10   |20   |30   |  
|A         |10   |20   |30   |  
|B         |100  |200  |     |  
|C         |25   |35   |40   |  
|D         |1000 |     |     | 
|E         |     |     |9999 |

For the final product, I need this entered in a new table 1 code at a time, so like so:

|ForiegnKey|Sequence|Code|
|A         |1       |10  |
|A         |2       |20  |
|A         |3       |30  |
|B         |1       |100 |
|C         |1       |25  |
|C         |2       |35  |
|C         |3       |40  |
|D         |1       |1000|
|E         |1       |9999|

For my question, is there a simple way to do this one in query that doesn't involve using unions?
I have to generate the sequence number based on the Value of the ForiegnKey (ie in the Case of ForiegnKey E, even tho it was in Column Code3, I need it to have a Sequence of 1.) My current thought path is that if I can get this out of the table without using a Union, I could generate the Sequence ID in the same statement.
Empty code values such as D-2 and 3, and E-1 and 2 should be skipped.
I have started working on this using a temporary table but thought I would check here and see if there was an easier way.

Thanks

Answer 1

You sound more interested in getting the results in one batch than in avoiding UNIONS. Here you go:

set @last_key = null;
set @seq = 1;
select foreignkey, case when @last_key is null or @last_key != foreignkey then @seq := 1 else @seq := @seq + 1 end as sequence, code, @last_key := foreignkey as foo  from (
  select foreignkey, code1 as code
  from dataset
  where code1 is not null
  union
  select foreignkey, code2
  from dataset
  where code2 is not null
  union
  select foreignkey, code3
  from dataset
  where code3 is not null
  order by 1
) foo;